The use of an exhaust gas sensor such as zirconia dioxide oxygen sensor as a means of deriving a feedback control signal for controlling the air-fuel mixture ratio at a desired point is well known for allowing a three-way catalytic converter to operate its maximum conversion efficiency, thus minimizing the amount of noxious waste products. Such oxygen gas sensor however exhibits a very high internal impedance when temperature in the exhaust system is very low during the warm-up period of the engine and thus the voltage derived from the gas sensor cannot be used to derive a valid feedback control signal. The usual practice is to detect the low temperature condition to suspend the closed loop operation until the gas sensor temperature reaches its normally operating level. Therefore, the noxious emissions are exhausted during such engine start periods.